<!DOCTYPE html>
<html lang="en">

    <head>
        <meta charset="UTF-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Document</title>
    </head>

    <body>
        <p>Name: <input type="text" data-help="Your full name"></p>
        <p>Age: <input type="text" data-help="Your age in years"></p>
        <p id="help"></p>

        <script>
            let help = document.querySelector("#help");
            let fields = document.querySelectorAll("input");
            for (let field of Array.from(fields)) {
                field.addEventListener("focus", event => {
                    let text = event.target.getAttribute("data-help");
                    help.textContent = text;
                });
                field.addEventListener("blur", event => {
                    help.textContent = "";
                });
            }
        </script>
    </body>

</html>